text: The traditional electrowetting mechanism has been receiving increasing interest due to its ability to control tension forces on a liquid droplet.  As surface tension acts as the dominant liquid actuation force in nano-scale applications, electrowetting has been used to modify this tension at the solid-liquid interface through the application of an external voltage. The applied electric field causes a change in the contact angle of the liquid droplet, and in turn changes the surface tensions across the droplet.  Precise manipulation of the electric field allows control of the droplets. The droplet is placed on an insulating substrate located in between an electrode.cooxoxc9x
  The optoelectrowetting mechanism adds a photoconductor underneath the conventional electrowetting circuit, with an AC power source attached.  Under normal (dark) conditions, the majority of the system's impedance lies in the photoconducting region, and therefore the majority of the voltage drop occurs here. However, when light is shined on the system, carrier generation and recombination causes the conductivity of the photoconductor spikes and results in a voltage drop across the insulating layer, changing the contact angle as a function of the voltage.  The contact angle between a liquid and electrode can be described as:
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  where V, d, ε, and γ are applied voltage, thickness of the insulation layer, dielectric constant of the insulation layer, and the interfacial tension constant between liquid and gas.  In AC situations, such as OEW, V is replaced with the RMS voltage.  The frequency of the AC power source is adjusted so that the impedance of the photoconductor dominates in the dark state.  The shift in the voltage drop across the insulating layer therefore reduces the contact angle of the droplet as a function of the light intensity.  By shining an optical beam on one edge of a liquid droplet, the reduced contact angle creates a pressure difference throughout the droplet, and pushes the droplets center of mass towards the illuminated side.  Control of the optical beam results in control of the droplets movement.
  Using 4 mW laser beams, OEW has proven to move droplets of deionized water at speeds of 7mm/s.
  Traditional electrowetting runs into problems because it requires a two-dimensional array of electrodes for droplet actuation.  The large number of electrodes leads to complexity for both control and packaging of these chips, especially for droplet sizes of smaller scales. While this problem can be solved through integration of electronic decoders, the cost of the chip would significantly increase.